1. Field of Invention
The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material capable of producing images with excellent preservation stability and/or improved chemical resistance to plasticizers or materials containing fats or oils.
2. Discussion of the Background
With diversification of information and expansion of needs therefor in recent years, various types of recording materials have been developed and put into practice in the field of information recording. Thermosensitive recording materials are widely used in various fields such as information processing (output of desk-top calculators, computers or the like), recorders for medical measurement devices, low- or high-speed facsimiles, automatic ticket machines (railway tickets, admission tickets or the like), thermal copying machines, labels of a POS (point of sales) system, and luggage tags because of the following advantages:
The thermosensitive recording materials have been used for applications in which reliability of images is required, such as POS systems and goods such as boxed lunches and daily dishes.
For example, the recording materials have been used for applications in which the recording materials are required to have good preservation stability to plasticizers and materials containing oils and fats; and applications in which the recording materials are required to have storage stability for several years.
The level of the requirements for the thermosensitive recording materials becomes higher and higher year by year. Therefore, attempts have been made to provide combinations of components including color developers and additives such as preservation stabilizers, to meet these challenges. However, materials having a well-balanced combination of coloring sensitivity and image preservation stability have not yet been developed.
Compounds of high molecular weight have been proposed for use as developer to particularly provide high preservation stability against plasticizers and materials containing fats and/or oils. The following compounds are examples of such developers: High molecular weight compounds based on diphenylsulfone derivatives (Japanese Laid-Open Patent Publication No. H08-333329); (poly) 4-hydroxybenzoic acid derivatives (WO99/51444); and Urea urethane compounds of high molecular weight (Japanese Laid-Open Patent Publication No.2000-143611).
As for these developers, preservation stability is high against plasticizers and materials containing fats and/or oils. However, they have the problem that coloring sensitivity is low. For example, the combination of a color developer of high molecular weight based on diphenylsulfone derivatives and a sensitizer of low melting point has been proposed in order to supplement coloring sensitivity. (Japanese Laid-Open Patent Publication No. H10-297089 and Japanese Laid-Open Patent Publication No. H10-297090)
These compositions attempt to give improved image recording density in low energy regions of the recording material by using the combination of a developer and a sensitizer having a low melting point. While coloring sensitivity of the top layer of the recording material is improved, there is found to be a significant drop in the image density due to decreased chemical resistance, such as to plasticizers. This shows the difficulty in obtaining both improved coloring sensitivity and improved image preservation and chemical resistance when using a color developer of high molecular weight.
Another problem that occurs upon improving coloring sensitivity is that background coloring begins to occur at lower temperatures. One solution that has been proposed is the use of a diphenylsulfone crosslinking type compound as a color developer, combined with an aromatic compound having an aminosulfonyl group (—SO2NH2) as sensitizer. (Japanese Laid-Open Patent Publication No. 2001-135867). However, while enough sensitivity was provided, the increased sensitivity was not compatible with improved heat resistance.
Another approach that has been suggested in order to solve these problems is to prevent coloring of a background while having improved sensitivity by using a combination of a low molecular weight color developer along with 4-hydroxy-4′-allyloxy diphenylsulfone as sensitizer, to provide heat resistance and sensitivity and chemical resistance (Japanese Laid-Open Patent Publication No. 2001-310561).
In addition, an approach has been proposed using an emulsification technique to provide a dispersion, with dispersion control of a leuco dye for using thermal energy from a thermal head. A heat insulation effect is provided by use of an undercoat layer including submicron sized hollow particles. However, sensitivity is not sufficient and background coloring occurs.